1. Field of the Invention
This invention pertains to the storage or backup of data using magnetic tape drives, and particularly to determining the wear of tape upon which data is stored/retrieved by such drives.
2. Related Art and Other Considerations
In magnetic recording on tape using a magnetic tape drive, relative motion between a head unit (typically with both a write element and a read element) and the tape causes a plurality of tracks of information to be transduced with respect to the tape. The magnetic tape is typically housed in a cartridge which is loaded into the tape drive. The tape extends between a cartridge supply reel and a cartridge take-up reel. The tape drive typically has a supply reel motor for rotating the cartridge supply reel and a take-up reel motor for rotating the cartridge take-up reel.
After the cartridge is loaded into the tape drive, the tape is extracted or loaded by mechanisms in the drive so that a segment of the tape is pulled from the cartridge and into a tape path that travels proximate the head unit. In some tape drives the extraction mechanisms take the form of tape guides which are mounted on trolleys. During the extraction operation, trolley motors move the trolleys along a predefined trolley path, so that the tape guides which surmount the trolleys displace the tape into the tape path as the trolleys travel along the trolley path. When the trolleys reach the full extent of travel along the trolley path, the tape is proximate the head unit. Thereafter the tape can be transported past the head unit, e.g., by activation of a capstan and/or the supply reel and take-up reel motors, depending upon the particular type of transport mechanisms employed. A capstanless tape drive, particularly a tape drive which utilizes helical scan recording, is shown in U.S. Pat. No. 5,602,694 for CAPSTANLESS HELICAL DRIVE SYSTEM, which is incorporated herein by reference.
As the tape is transported past the head unit, information can be transduced to or from the tape by the tape drive in recording and reading operations, respectively. When the recording and/or reading operations are concluded, and before the cartridge can be unloaded from the drive, the tape must be retracted for return to the interior of the cartridge. Tape retraction is essentially the reverse of the tape extraction procedure described above.
Because removable media such as that housed in a tape cartridge can be reused many times, the issue of when a magnetic tape is worn out assumes great importance to the end user. If a tape is near its end of life, data errors become more common and the possibility of a catastrophic loss increases. Manufacturers of computer tape drives typically specify tape durability in terms of a maximum allowable number of "passes" of the head unit over the tape. In the simplest case, writing data to a tape might involve only two passes over a given spot of the tape: a first pass in the forwarded direction to write the data and a second pass in the reverse direction to return to the beginning of the tape prior to an unload of the cartridge. In practice, issues of formatting, data rate mismatches, and error recovery cause the tape motion to be much more complex. Certain sections of the tape might be subject to many passes during a given operation while nearby sections see much less activity. Measuring tape wear under such real world circumstances has traditionally been impossible. As a compromise, many tape drive systems instead measure other factors such as the number of rewinds or load cycles which are only indirectly related to the number of passes.
For example, as previously marketed, the Exabyte Mammoth.TM. tape drive attempts to determine remaining tape life indirectly by recording how often certain types of motion have occurred on a tape. However, no record is made of where these motions have occurred on the tape. A relatively small number of start/stop motions occurring in a small area of tape can wear out that section, whereas a much larger number of similar motions will not do so if spread out across a large section of tape.
Other approaches for discerning tape life are provided in the prior art. U.S. Pat. No. 4,797,753 to Montgomery employs a separate calibration track whose output level and frequency content is monitored for degradation. U.S. Pat. No. 4,575,778 to Vogelgesang describes a mechanical counting wheel which is advanced one per use of a cassette. U.S. Pat. No. 4,485,674 to Ragle teaches that a so-called "stagger wrap" profile of a reel is unique for each rewind cycle, and thus can be used to indicate the number of cycles.
What is needed, therefore, and an object of the present invention, is a technique for effectively and efficiently indicating tape wear.